1. Field of the Invention
The present invention relates to a television receiver or a monitor, and more particularly, to an automatic beam limiter circuit capable of preventing the rapid increase of beam current when a black field is rapidly changed into a white field.
2. Description of Related Art
Cathode ray tubes (CRTs) are commonly used in television sets. In a CRT, beam current flows from an anode to a cathode and increases in proportion to the amplitude of video signals input to the cathode. When beam current flowing through a CRT exceeds the rated current of peripheral circuits such as a fly back transformer (FBT) and a CRT driver, in any field, the peripheral circuits can be damaged or image displayed on the CRT distorted. Thus, the CRT should maintain an appropriate brightness level in response to a video signal input. In most television receivers, an automatic beam limiter (ABL) circuit or an automatic brightness limiter circuit is used to control the amount of beam current.
The ABL circuit senses beam current flowing in fields and automatically reduces brightness and/or contrast when the amount of sensed beam current exceeds a predetermined limit. As such, the ABL circuit protects a CRT by automatically reducing the amplitude of video signals input to the cathode of the CRT and prevents beam current overflow.
FIG. 1 is a block diagram of a television receiver including a conventional ABL circuit. Referring to FIG. 1, the television receiver includes a brightness/contrast control circuit 2, an RGB matrix 4, a digital-analog converter (DAC) 6, a CRT driver 8, a CRT 10, a fly back transformer (FBT) 12, a beam current detection circuit 14, and an ABL circuit 20.
The ABL circuit 20 includes an analog-digital converter (ADC) 22, a field average circuit 24, a calculator 26, and a gain control circuit 28.
The brightness/contrast control circuit 2 controls the brightness and/or contrast of a video signal IN input therein in response to a gain control signal Gout output from the gain control circuit 28, and outputs the controlled video signal IN, as a brightness signal (Y signal) and a chroma signal (C signal) to the RGB matrix 4.
The RGB matrix 4 outputs red, green, and blue signals in response to the output signals (brightness and chroma signals) of the brightness/contrast control circuit 2. The DAC 6 converts digital signals output from the RGB matrix 4 into analog signals and outputs the analog signals to the CRT driver 8.
The CRT driver 8 amplifies the analog signals output from the DAC 6 and inputs the analog signals into the cathode of the CRT 10. The CRT 10 is a vacuum tube for displaying the video signal IN on a screen of a television.
The FBT 12 outputs beam current ib to the anode of the CRT 10 in proportion to the level of the video signal IN input to the cathode of the CRT 10 and generates a high voltage of several kilovolts or greater required in the anode of the CRT 10.
The current detection circuit 14 detects the beam current ib output from a power supply voltage VCC to the cathode of the CRT 10 through resistors R1 and R2 and the FBT 12 and outputs a detection signal corresponding to the detected beam current ib to the ADC 22. A capacitor C removes switching noise from the FBT 12.
The ADC 22 of the ABL 20 converts an analog signal at a node VA to an N-bit digital signal and outputs the N-bit digital signal to the field average circuit 24. The field average circuit 24 calculates the average of the output signal of the ADC for a predetermined frame or a predetermined field in response to a synchronous signal Vsync and outputs the average to the calculator 26.
The calculator 26 can be used as an adder and a subtractor. The calculator 26 calculates the difference between a reference value REF and the output value of the field average circuit 24, that is, an average beam current Fave and outputs the calculated signal to the gain control signal 28.
The gain control circuit 28 outputs the gain control signal Gout, which controls the brightness and contrast of the video signal input to the CRT 10, to the brightness/contrast control circuit 2 in response to the output signal of the calculator 26. As a result, if the brightness of a CRT screen increases, the beam current ib increases. Then, the ABL 20 reduces the brightness and contrast of the video signal IN input to the CRT 10 to decrease the level of the video signal IN of the CRT 10.
However, because the conventional ABL 20 takes the average of the beam current ib for every field and adjusts the beam current ib for field, as illustrated in FIG. 5(b), at least one field is delayed.
As a result, when a black field is rapidly converted into a white field as shown in FIG. 5(a), an excessive amount of beam current ib flows in one field as illustrated in FIG. 5(b). Therefore, when averaging the beam current ib for every field, as described in the conventional ABL 20, the CRT 10 and its peripheral circuits receive a spiked current during one field period, and noise occurs at the FBT 12.